Ink jet printing methods are well known for application of compositions to a wide range of substrates. Ink jet printing methods are versatile and particularly valuable for printing indicia on substrates used in high speed processes. Examples of such processes include processes of packaging food products in which batch and date information is applied to the packaging. In processes in which the packaging is formed from a polymeric film (e.g. sweet wrappers) the film passes rapidly from the printing station to a position at which the product is wrapped. In such processes it is important that the ink composition is substantially dry before the wrapping process begins, since if it is not then smudging or even removal of the printed indicia can occur. It is also important that the ink has a composition which adheres well to the substrate, since if it does not, in this case also smudging and removal can occur.
Particular problems arise with substrates which are based upon polymers of monomers which include propylene (polypropylenes) or polymers of monomers which include ethylene (polyethylenes). Adherence to these substrates is difficult and is made more difficult when, as is common, they are treated with a release agent (often necessary for subsequent processing of the film to form a product wrapping).
Therefore it would be desirable to be able to provide an ink composition which is capable of rapid drying and is also capable of being formulated to include binder materials which give good adhesion to substrates, in particular the polypropylene- and polyethylene-based substrates mentioned above.
It has been found in the past that solvent components such as methyl ethyl ketone (MEK) and acetone have high drying rate and are thus suitable for use as solvents in compositions which are required to dry rapidly. However, it has been well known for many years that compositions which are formulated to dry rapidly on application to the substrate can give other problems. In particular, rapidly drying ink compositions can lead to problems with blocking of the nozzles, and ink drying in the gutters, of the printing equipment used to apply the ink to the substrate. For this reason inks which include fast drying solvents such as MEK have routinely included humectants and additional solvents known to have higher boiling point, in order to prevent this, which in turn slows down the drying time.
It would be desirable to be able to solve the above mentioned drying and adhesion problems without introducing additional problems of blocking and crusting in the printing equipment. WO92/14794 is concerned with the problem of providing a rapidly drying ink which dissolves commonly-used dyestuffs and is particularly concerned with the problem of replacing the commonly-used solvent MEK. It is also concerned with providing an ink having suitable viscosity. WO92/14794 uses a carrier medium comprising a lower alkyl ester of a lower alkyl carboxylic acid and generally also a lower alkanol. Alkyl moieties for these materials of length C1 to C5 are mentioned generally but the majority of the examples include ethyl acetate, which is the highly preferred alkyl ester for use in WO92/14794. Ethyl butyrate and butyl acetate are also used in other examples.
U.S. Pat. No. 5,990,197 discloses ink formulations generally containing a near infrared fluorophore copolymerised with a polyester, a binder, an organic solvent soluble electrolyte and an organic solvent selected from a C3–C6 aliphatic ketone, a C3–C6 aliphatic ester, a lower aliphatic alcohol or combinations thereof. The example inks use 2-butanone or ethanol or a mixture of the two as the solvent. Methyl acetate is mentioned in a list of C3–C6 aliphatic esters but of these ethyl acetate is said to be preferred.
GB-A-2030075 discloses an ink jet printing process using opaque inks. The ink contains resin or polymer dissolved in a solvent. A wide range of solvents is disclosed, and a list is given reciting aliphatic alcohol, ketones, aldehydes, ethers, esters, glycols, glycol ethers, hydrocarbons and lactones. Methyl acetate is mentioned in a list of ester-type solvents but in the examples the solvents used are methanol or mixtures of methanol with MEK or mixtures of methanol, MEK and acetone.